Print lithops resources are not numerous and, as it may seem at first sight, often unavailable.

First of all, check if a library near you has the book by looking it up in the global library catalog WorldCat (you will need to enter your location). Even if your local library does not have it, ask them whether it would be possible to request the book through inter-library loan. As far as articles are concerned, you may be able to access full-text online journals on the topic (such as Cactus and succulent journal) through your local public or university library. Finally, check Amazon or eBay to see if you can get hold of a used copy of a book, which should be much cheaper than buying a new one.